1.0 - Measuring Variability

Precision Agriculture (PA) aims to improve a grower’s ability to manage within-field variability. PA provides practitioners with tools to quantify soil, terrain, and crop variability and thereby customise agronomic practices and fine-tune resource applications to better match these variables.

Variability can be both spatial and temporal.

Spatial Variability: The variation found in soil, terrain, and crop properties across an area at a given time. For example soil pH and crop yield.

Temporal Variability: The variation found in soil and crop properties within a given area at different measurements in time. For example the difference in yield maps from one season to the next. There are many factors that contribute to the spatial and temporal variability within a field.

Some of these factors may include: 

• Soil Attributes: Soil pH, texture, structure and depth, soil organic matter, soil water, soil chemistry, and subsoil constraints. 
• Terrain: Soil forming processes, natural or formed elevation, depressions, aspect, and slope. 
• Management Practices: Cropping practices (e.g. control traffic farming) and management history (e.g. strategic direction of farming, crop rotations).
• Environmental Factors: Weather, weed, insect, and disease.

The magnitude of these factors will influence the degree of variability within a field and the feasibility of managing that variability.

Wheat yield map showing the spatial variability in yield with the accompanying histogram of this data. 

Steps for managing variability:

• Identify and measure the variability and quantify the variability on both a spatial and temporal scale.

• Investigate the cause of the variability.

• Assess strategies to optimise the management of the variability.

Managing variability, where to begin:

• Collect, compile, and utilise spatial data. Keep it simple. Begin by logging good data; organizing, storing, and backing it up in a systematic manner. Be meticulous about documenting events within the farm operation.

• Quantify yield variability (magnitude and spatial distribution) using yield monitors to generate accurate and high-quality data. EM and subsoil constraints can be identified by an electromagnetic (EM) survey followed by appropriate soil sampling.

• Terrain can be assessed by collecting real-time kinematic (RTK) elevation data which can be used to generate a variety of elevation derivatives such as slope, aspect, and wetness index.

• Combine and compare data layers where appropriate. For example: – Assess the temporal variability of a field by comparing the yield data over several years; and, – Examine the spatial variability of a field by comparing the appropriate elevation derivatives with yield data to determine the impact of terrain on yield.

• Compare ground-truthed soil layers such as EM surveys with yield data to assess the impact of surface and subsoil variability. For example changes in clay content will influence water-holding capacity and yield potential.

• Utilise expert grower knowledge to help explain observations in the spatial data.

• Generally, it is more instructive to compare yield data from the same growing season (i.e. winter yield maps with other winter yield maps and summer yield maps with other summer yield maps).

• Seasonal variability can have a subsidiary effect on consecutive crops. The amount of nitrogen fixed by a legume crop may be variable creating a natural variable rate application for the next crop.

• Critically assess agronomic practices: – Can weed, disease, and pest pressures be reduced with alternate management strategies? – Are yield and quality goals in line with current fertilizer inputs?

• Identifying the reason for observed variability will enable the appropriate management options to be considered. These options will be specific to the resources and goals of the individual and must be balanced against any environmental considerations.